This gives a 10% allocation improvement on MultiLayerModules.
The idea is to reuse existing tuples, instead of constantly
constructing new ones.

Test Plan: ./validate

Reviewers: simonpj, bgamari

Reviewed By: simonpj, bgamari

Subscribers: rwbarton, thomie, simonmar, carter

Differential Revision: https://phabricator.haskell.org/D4332